High power traveling-wave tube



Sept. 9,1958 c. K. BIRDSALL 2,851,630

HIGH POWER TRAVELING-WAVE TUBE Filed April 13, 1955 7 2 Sheets-Sheet 1FIG.

CHARLES K BIRDSALL INVENTOR ATTORNEY INPUTK c. K. BIRDSALLY HIGH POWERTRAVELING-WAVE TUBE Sept. 9,

2 Sheets-Sheet 2 Filed" April 15, 1955 FIG. 2

M m R T S N 0 R T C m PITCH.

HELIX P3, & P4

mm L E H 7 I v f F a {5, FRE uENcY- G N V F mm H E W B H A U G T H 3 CErr- V M W G DG. F RN um m w emu A R .BTIA 0 ID FIG. 4,

FREQUENCY CHARLES K. BlRDSALL INVENTOR ATTORNEY United States PatentHIGH POWER TRAVELING-WAVE TUBE Charles K. Birdsall, Los Angeles, Calif.,assignor to Hughes Aircraft Company, Culver City, Calif., a corporationof Delaware Application April 13, 1955, Serial No. 501,212

Claims. (Cl. 315-35) This invention relates to high power microwavetubes and more particularly to a traveling-wave tube incorporating alarg? diameter helical slow-wave structure which possessescharacteristics that greatly increase the startoscillation current forbackward-wave oscillations.

As is presently known, it is advantageous in a travelingwave tubeincorporating a helix to have a helix circumference of the order of ahalf wavelength at the operating frequency of the device in order toachieve very high power outputs at the higher frequencies. Helices thislarge allow very large beam currents to be employed at practical currentdensities. A tube with a large diameter helix is disclosed in acopending application for patent Serial No. 401,303, entitledTraveling-Wave Tube" filed by Dean A. Watkins and Horace R. Johnson, onDecember 30, 1953. In this type of tube, however, it is necessary toprovide means for preventing the device from commencing backward-waveoscillations, i. e., to increase the start-oscillationcurrent forbackward-wave oscillations.

It has been found that slight variations in the pitch of the helix in atube of the aforementioned type may have little or no effect on itsforward-wave gain characteristics but may have ,a very pronounced effecton the startoscillation current for backward-wave oscillations. By wayof example, a slight change in the pitchof the turns for one-half thelength of a large diameter helix with respect to the pitch of the turnsof the remaining half may increase the start-oscillation current byapproximately .8 times that of helix having a uniform pitch along itsentire length. It is thus apparent that with the pitch change, the beamcurrent employed for forward-wave amplification in such a device may besubstantially increased without having backward-wave oscillationscommence, as would have been the case with uniform pitch.

In accordance with the present invention, a high power traveling-wavetube is provided that incorporates a periodic slow-wave structure suchas, for example, a large diameter helix, which has a plurality ofsections of different pitch. Alternatively, in order to avoidreflections due to the abrupt changes in pitch, a slow-wave structuresuch as a large diameter helix, having either a progressively increasingor decreasing pitch is employed. In this manner, the start-oscillationcurrent for backward-wave oscillations is increased to the extent thatthe device may be used effectively as a forward wave amplifier withoutother means of preventing the backward-wave oscillations.

It is therefore an object of this invention to provide an improved highpower traveling-wave amplifier tube.

Another object of this invention is to provide a traveling-waveamplifier tube incorporating a helical slowwave structure requiiingsubstantially more current to commence backward-wave oscillations thanin conventional structures of this type.

Still another object of this invention is to provide a traveling-wavetube incorporating a helical slow-wave structure constituted of aplurality of sections having dif- 2,851,630 Patented Sept. 9, 1958ferent pitch to prevent backward-wave oscillations from commencing.

A further object of this invention is to provide a traveling-wave tubeincorporating a helical slow-wave structure having either aprogressively increasing or decreasing pitch to prevent backward-waveoscillations from commencing.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which several embodiments of the invention areillustrated by way of example. It is to be expressly understood,however, that the drawings, are

for the purpose of illustration and description only, and are notintended as a definition of the limits of the invention.

Fig. 1 is a diagrammatic sectional view in partial elevation of anembodiment of the invention together with associated circuitry;

Fig. 2 is an elevational view of an alternate embodiment of the helicalslow-wave structure of the present invention; and

Figs. 3 and 4 are illustrative diagrams of gain versus frequency.

Referring now to the drawings, Fig. 1 illustrates an embodiment of thepresent invention comprising an evacuated envelope 10 which consists ofa long cylindrical portion 11 and an enlarged portion 12 at the leftextremity, as viewed in the drawing. Enlarged portion 12 houses anelectron gun 14 which produces a hollow cylindrical elec tron stream.This electron stream is directed concentrically through the longcylindrical portion 11 to the opposite extremity thereof by a solenoid15 where it is intercepted and collected by a collector electrode 16. Inaccordance with a preferred embodiment of the invention, a helicalslow-wave structure 18 having a non-uniform pitch and provided withcoaxial input and output cables 20 and 22, respectively, is disposedcontiguously along the path of the electron stream intermediate theelectron gun 14 and the collector electrode 16.

More particularly, electron gun 14 comprises an annular cathode 24 witha heater 25 and an electron emitting surface 26, a focusing electrode23, and an accelerating electrode 30, the electrodes 28 and 30 beingprovided with apertures in register with the electron emitting surface26 of cathode 24 to allow passage therethrough of the hollow cylindricalelectron stream. The electron emitting surface 26 is energized by meansof a battery 32 connected across the heater 25, one terminal of whichmay be connected to the cathode 24, as shown. The focusing electrode 28provides an inner and an outer surface of revolution adjacent to andabout the electron emitting surface 26 of cathode 24 at an angle ofapproximately 67.5 with the longitudinal axis of the helix 18, as shownin the drawing. Cathode 24 and focusing electrode 28 are connectedtogether and are maintained at a potential of the order of 3000 voltsnegative with respect to ground by means of a connection therefrom tothe negative terminal of a battery 34, the positive terminal of which isconnected to ground.

Accelerating electrode 30 is disposed in a plane normal to thelongitudinal axis of helix 18 and to the right of focusing electrode 28,as viewed in the drawing. Electrode 30 is maintained at a potential thatis of the order of 200 volts positive with respect to ground by means ofappropriate connections to a battery 36. An equipotential region isprovided between the electron gun 14 and the helix 18 by means of aconductive coating 38 disposed about the inner periphery of the envelope10 intermediate the focusing electrode 28 and the helix 18. This '3equipotential region is maintained at the same potential as theaccelerating electrode 30 by means of a connection to the battery 36.

The hollow electron stream thus produced by electron gun 14 is directedconcentrically through the helix 18 by means of the solenoid 15 which isdisposed concentrically about the envelope and is coextensive therewith.An appropriate direct current is made to flow through the solenoid toproduce an axial magnetic field of the order of 600 gauss to constrainthe electron stream throughout the active length of the tube. Thesolenoid 15 is thus energized by means of connections across a battery38. The collector electrode 16, disposed so as to intercept the electronstream at the right extremity of envelope 10, as viewed in the drawing,is maintained at a potential more positive than that of helix 18 tosuppress secondary electron emission. In the present case, this isaccomplished by maintaining collector electrode 16 at a potential of theorder of 50 volts positive with respect to ground by means ofappropriate connections to a battery 40.

Disposed intermediate the electron gun 14 and the collector electrode 16is the helical slow-wave structure 18. An electromagnetic signal wave islaunched on the helix 18 by means of the input coaxial section 20.Coaxial section 20 has a center conductor 42 which connects the highside of input terminals 44 directly to the first turn of helix 18nearest the electron gun 14. A resistor 46 is interconnected between thecenter conductor 42 of coaxial section 20 and ground in order tomaintain the helix 18 at quiescent ground potential. The outer conductorof coaxial section 20 is connected to a ferrule 48 which is disposedconcentrically about the first several turns of the helix 18 on theoutside of the envelope 10. The extremity of ferrule 48 away fromelectron gun 14 is flared out in order to improve the impedance matchbetween coaxial section 20 and helix 18. The outer conductor of coaxialsection 20 and ferrule 48 may be maintained at any substantially fixeddirect-current potential such as, for example, at ground by means of aconnection thereto.

The output from the device of the present invention is provided by theoutput coaxial section 22. Section 22 has a center conductor 50 whichinterconects the last turn of helix 18 farthest from the electron gun 14to the high side of output terminals 52. In addition, a matching ferrule54 is disposed concentrically about the last several turns of helix 18and connected to the outer conductor of coaxial section 22 which is, inturn, connected to ground in the same manner as is that of the inputcoaxial section 20.

As previously specified, the helical slow-wave structure 18 is of thetype that has a large circumference as compared to a free spacewavelength at its operating frequency, such as, for example, 0.2 toone-half a free space wavelength. The inner diameter of the helicalstructure 18, on the other hand, is preferably of the order of 1.4 timesthe diameter of the hollow cylindrical electron stream produced byelectron gun 14. 'In accordance with a first embodiment of theinvention, the length of helical structure 18 is constituted of aplurality of portions, each of which has a different pitch. As shown inFig. 1, for example, the portions a, 1;, c, and d of the helicalstructure 18 have pitches of p 2 p and p turns per inch, respectively.Alternatively, the pitch of the helical structure may be, in accordancewith a second embodiment of the invention, made to progressivelyincrease or decrease as represented by the helix 18a, shown in Fig. 2.The helix 18 or 18a maybe reversed, end for end, with respect to thedirection of the electron stream, in which case the helix pitch wouldincrease or decrease, respectively.

erated. For example, a typical backward-wave gain versus frequencycharacteristic 60 for a traveling-wave tube incorporating a helix havinga uniform pitch is illustrated in Fig. 3. Referring to this figure, itis seen that the gain of a conventional tube increases sharply at thefrequency,

This frequency, f is dependent upon the circumference and pitch of thehelix and the velocity of the electron stream. It is thus apparent that,with a gain of this magnitude at the frequency i there would be atendency for the tube to commence backward-wave oscillations as it onlyremains to satisfy the phase requirements for oscillation about theincremental feedback loop along the helix when the total gain about theequilavent feedback path is greater than unity.

However, it is possible to satisfy the conditions for backward-waveoscillations for only a portion of the length of the slow-wavestructure. Hence, the backwardwave attenuation for a particularfrequency over the remaining portion of its length cannot be relied uponto prevent backward-wave oscillations. Referring to Fig. 4, there areillustrated the gain characteristics for a constant velocity electronstream of the helix 18 having pitches 121, 2 p and p and helix 18ahaving a progressively increasing or decreasing pitch, represented bydashed lines 62, 64, respectively. The gain characteristic 62 of helix18, for example, has backward-wave gain at frequencies f f ,f and fwhich correspond, respectively, to the pitches 1 1 p and The gain at anyof the frequencies f f f and 1, may be of the order of l/ 64 that of asingle helix having a single uniform pitch. Also, the gaincharacteristic 64 of the helix 18a extends over a comparatively broadportion of the frequency spectrum and may be of the order of l/ 20 thatof a single uniform pitch helix, depending on the extent of the increaseor decrease of pitch over the length of the helix. The forward wavegain, on the other hand, as represented by line 66 of Fig. 4, extendsover a broad range of frequencies and is not substantially affected bythe different pitches p 2 p and p of helix 18 or the progressivelyincreasing or decreasing pitch of helix 18a. Thus, it is seen that thehelices 18 or 18a of the present invention may be employed as theslow-wave structure in a traveling-wave amplifier tube to substantiallydecrease the start-oscillation current for backward-wave oscillations.

More particularly, it may be shown that the conditions for backward-waveoscillations for a uniform pitch or periodic structure are met when thefollowing relation exists:

CN 0.3 l4 l wherein C is the Pierce gain parameter of the device, and Nis the total length of the uniform pitch or periodic slowwave structurein Wavelengths.

The Pierce gain parameter, C, however, is proportional to the one-thirdpower of the beam current, i. e.

wherein I is the current represented by the electron beam producedbyelectron gun 14. It is thus apparent that if I be designated as thestart-oscillation current, i. e., the

current at which backward-wave oscillations commence, then I N=constant(3) ance with the device of the present invention, the length of theslow-wave structure is divided into n portions, prefer'ably of equallength, having different pitches. In this case, the Relation 3 for thecommencement of backwardwave oscillations becomes I =c0nstant and fromthe ratio of Relations 3 and 4 An additional consideration is the extentof the allowable variation between the pitches p p p and 12 In order toascertain the maximum allowable variation in pitch, consideration mustbe given to the forward-wave gain characteristics of the device. Ingeneral, it can be shown that the forward-wave gain is down 3 decibelsfrom its maximum where the ratio of the greatest to the smallestvelocity by which the slow-wave structure propagates an electromagneticwave is approximately equal to the quantity (1+2C) where, as before, Cis the Pierce gain parameter. In the case of the helices 18 and 1801,the velocity at which an electromagnetic wave is propagated is directlyproportional to the pitch. Thus p largest p smallest (6) In the case ofa high power traveling-wave amplifier tube, a typical value of C is 0.1or larger. Hence the ratio of the largest to the smallest pitch shouldbe equal to or less than 1.1.

On the other hand, in order to determine the closeness with which thepitch of two different portions of the slow-Wave structure may differ,it is necessary to consider the variation of frequency with pitch. Itmay be shown that a 1% pitch variation is sufficient to shift thefrequency, f at which maximum gain occurs by at least 1%. In that thewidth of the portion of the frequency spectrum amplified is only of theorder of 0.2%, it is evident that a 1% change in frequency and hencepitch is more than adequate. Thus, from the foregoing it may be statedthat the ratio of the pitch for any section of the slow-wave structureto the smallest pit-ch should be within the range from 1.01 to 1.10. Itis, of course, desirable to make the changes in pitch as small aspracticable and still satisfy the above relation so as to effect maximumforward wave gain and in addition make the changes occur over atransition region so as to minimize reflections.

What is claimed is:

1. A traveling-wave amplifier tube comprising a helical slow-Wavestructure having a plurality of different pitches and a circumferenceper turn of not less than 0.2 free space wavelength at the mid-frequencyof operation of the tube, means for launching an electromagnetic signalwave along said helical slow-wave structure, means for producing ahollow, cylindrical electron stream having a cross-sectional area thatis small compared to the cross-sectional area of said helical slow-wavestructure, and means for directing said electron stream contiguouslyalong said helical slow-wave structure at a velocity to amplify saidsignal wave, whereby said different pitches of said helical slow-wavestructure substantially decrease the maxium overall backward-wave gainof said tube to minimize backward-wave oscillations, the inner diameterof said slow-wave structure being of the order of 1.4 times the diameterof said electron stream.

2. The traveling-wave amplifier tube as defined in claim 1 wherein saidplurality of different pitches of said helical slow-wave structure isprovided by a progressively increasing pitch in the direction ofelectron flow along said electron stream.

3. The traveling-wave amplifier tube as defined in claim 1 wherein saidplurality of different pitches of said helical slow-wave structure isprovided by a progressively decreasing pitch in the direction ofelectron flow along said electron stream.

4. The traveling-wave amplifier tube as defined in claim 1 wherein saidplurality of different pitches of said helical slow-wave structure isprovided by several portions of the length of said structure, eachhaving a different uniform pitch.

5. The traveling-wave amplifier tube as defined in claim 4 wherein theratios of the pitches of said portions to the minimum pitch thereof isfrom 1.01 to 1.1.

References Cited in the file of this patent UNITED STATES PATENTS UNITEDSTATES PATENT OFFICE CERTIFICATE @F 'COREQ Patent No, 2 851,, 630

Column 4,

Charles K. Birdsall line 32 3 for "'l/ZO' read 1' 1/120 o IHQN September9 1958 appears in the-printed specification correction and that the saidLetters Signed and sealed "this 18th day of November 1958 (SEAL) Attest:

KARL 1H,, AXLINE Attesting Officer ROBERT C. WATSON Commissioner ofPatents

